1. Field of the Invention
The present invention relates to a shift-drum apparatus for changing the speed of a gear-type transmission, and to a four wheeled vehicle with the shift-drum apparatus.
2. Description of the Related Art
FIG. 13 is a development view of a conventional shift-drum apparatus of a gear-type transmission. A plurality of bent shift grooves (cam grooves) 201 and 202 are formed in an outer peripheral surface of a shift drum 200, and drive pins of shift forks 203 and 204 are engaged with the shift grooves 201 and 202, respectively. When the shift drum 200 rotates, the shift forks 203 and 204 move in an axial direction (axial direction O2) of the shift drum 200 by a cam effect of side surfaces of the shift grooves 201 and 202, and a transmission gear stage of the gear-type transmission is shifted to a desired transmission gear stage.
More specifically, when the shift drum 200 is at a forward high speed position H, the one shift fork 204 moves toward one side in the axial direction (axial direction O2) of the shift drum 200, i.e., in a direction of an arrow K1 by the cam effect of the one shift groove 202, thereby coupling a corresponding dog clutch (not shown). When the shift drum 200 is at a forward low speed position L, the other shift fork 203 moves in a direction K1 by a cam effect of the other shift groove 201, thereby coupling a corresponding dog clutch (not shown). When the shift drum 200 is at a reverse position R, the other shift fork 203 moves toward the other side of the axial direction (axial direction O2) of the shift drum 200, i.e., in a direction of an arrow K2 by the cam effect of the other shift groove 201, thereby coupling a corresponding dog clutch (not shown).
However, in the shift operation, tip ends of dog teeth of corresponding dog clutches abut against each other in some cases. In such a case, it is difficult to swiftly complete the shifting action. Especially in the case of a vehicle having a centrifugal clutch, since power to the gear-type transmission is cut off at the time of idling and an input shaft of the gear-type transmission is stopped, the shifting action can not be completed in some cases.
Apart from the above-described shift-drum apparatus, Japanese Patent No. 2515958 discloses another shift-drum apparatus in which a shift drum is biased in a rotating direction by a spring so that a waiting function is exhibited. That is, even if tip ends of dog teeth abut against each other at the time of shifting operation, the waiting function is exhibited by the spring, and after both the dog teeth relatively rotate, the shift drum is rotated by the spring and the dog clutches are meshed with each other. However, the waiting function for biasing the shift drum in the rotating direction requires an extremely strong spring tension while taking, into account, inertial mass of the shift drum, friction of a rotation bearing and rotation speed, and an operation load of a shift operating lever is increased.
FIG. 14 shows a shift-drum apparatus having another waiting function described in Japanese Patent No. 251598. A gear-type transmission that is operated by this shift-drum apparatus can shift gears between neutral, a forward high speed stage, a forward low speed stage and a reverse stage. Two shift grooves 301 and 302 are formed in an outer peripheral surface of a shift drum 300.
A reverse cam portion (bent portion) 301R that is bent in an axial direction, and a forward low speed recess 301L that is depressed in the axial direction are formed in the one shift groove 301. A forward high speed cam portion 302H that is bent in the axial direction is formed in the other shift groove 302. A shift fork 303 that is engaged with the one shift groove 301 is biased toward the recess 301L (in the direction of an arrow K1) in the axial direction by a spring (not shown) and with this, the waiting function is exhibited when shifting to the forward low speed stage.
According to the shift drum 300 shown in FIG. 14, the one shift groove 301 has the forward low speed recess 301L having the waiting function, and the reverse cam portion (bent portion) 301R as described above, but since the forward low speed dog clutch and the reverse dog clutch are engaged and disengaged using the one shift fork 303, the shift drum has the following problem.
When shifting from the forward low speed stage to the reverse stage for example, it is necessary to once return the shift fork 303 to a neutral position N from the forward low speed position L and then to shift the shift fork 303 to the reverse position (301R). That is, it is necessary to secure a necessary rotating angle of the shift drum 300 in two stages, and a stroke of a driver's manual shift lever operation is increased, and if the operation stroke is shortened, an operation load is increased.